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Thème : Jamming, plasticity and material failure

Présentation

The constituents of jammed and glassy soft materials are so densely packed and/or so tightly bound that their microscopic dynamics are orders of magnitude slower than in dilute systems. Examples of such materials include, among others, concentrated colloidal suspensions or emulsions, polymer or particle gels, surfactant phases, foams. Our research focuses on the spontaneous (e.g. due to thermal energy) dynamics of these materials, as well as on their behavior in response to an external drive, e.g. a mechanical or thermal forcing.
We leverage on our expertise in the formulation of well-defined supramolecular systems with original properties, combined with home-designed devices allowing a thorough characterization of the structure and dynamics of the system and a fine control of the imposed drive.

Fractures propagating in a dense corn starch suspension. The suspension is confined between two parallel plates separated by a thin gap. The fracture is caused by the fast injection of solvent, at a point about 3 cm from the field of view. The field of view is about 1 cm2. The sample is illuminated using laser light : the temporal fluctuations of the resulting speckle patter carry information on the microscopic dynamics and the velocity field (see Photon Correlation Imaging in Glassy and Jammed Soft Matter). By Frederic Lechenault, Serge Mora and Luca Cipelletti.

To investigate the interplay between microscopic dynamics and macroscopic rheology insoft matter, we couple a stress-controlled-rheometer equipped with a Couette cell to a lightscattering setup in the imaging geometry, which allows us to measure both the deformationfield and the microscopic dynamics. To validate our setup, we test two model systems. For anelastic solid sample, we recover the expected deformation field within 1 μm. For a pure viscousfluid seeded with tracer particles, we measure the velocity profile and the dynamics of thetracers, both during shear and at rest. The velocity profile is acquired over a gap of 5 mm witha temporal and spatial resolution of 1 s and 100 μm, respectively. At rest, the tracer dynamicshave the expected diffusive behavior. Under shear, the microscopic dynamics corrected for theaverage drift due to solid rotation scale with the local shear rate, demonstrating that our setupcaptures correctly the relative motion of the tracers due to the affine deformation.

Rearrangement zone around a crack tip in a double self-assembled transient network

We investigate the nucleation and propagation of cracks in self-assembled viscoelastic fluids, which are made of surfactant micellesreversibly linked by telechelic polymers. The morphology of the micelles can be continuously tuned, from spherical to rodlike towormlike, thus producing transient double networks when the micelles are sufficiently long and entangled and transient singlenetworks otherwise. For a single network, we show that cracks nucleate when the sample deformation rate involved is comparable tothe relaxation time scale of the network. For a double network, by contrast, significant rearrangements of the micelles occuras a crack nucleates and propagates. We show that birefringence develops at the crack tip over a finite length, ξ, whichcorresponds to the length scale over which micelle alignment occurs. We find that ξ is larger for slower cracks, suggesting anincrease of ductility.

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Tuning the Structure and Rheology of Polystyrene Particles at the Air-Water Interface by Varying the pH

We form films of carboxylated polystyrene particles (C-PS) at the air water interface and investigate the effect of subphase pH on their structure and rheology by using a suite of complementary experimental techniques. Our results suggest that electrostatic interactions drive the stability and the structural order of the films. In particular, we show that by increasing the pH of the subphase from 9 up to 13, the films exhibit a gradual transition from solid to liquidlike, which is accompanied by a loss of the long-range order (that characterizes them at lower values of pH). Direct optical visualization of the layers, scanning electron microscopy, and surface pressure isotherms indicate that the particles deposited at the interface form three-dimensional structures involving clusters, with the latter being suppressed and a quasi-2D particle configuration eventually reached at the highest pH values. Evidently, the properties of colloidal films can be tailored significantly by altering the pH of the subphase.